Systems and Methods for Selecting Reconfigurable Antennas in MIMO Systems

ABSTRACT

A method allows reconfigurable multi-element antennas to select the antenna configuration in MIMO, SIMO and MISO communication system. This selection scheme uses spatial correlation, channel reciprocal condition number, delay spread and average Signal to Noise Ratio (SNR) information to select the antenna radiation pattern at the receiver. Using this approach, it is possible to achieve capacity gains in a multi-element reconfigurable antenna system without modifying the data frame of a conventional wireless communication system. The capacity gain achievable with this configuration selection approach is calculated through numerical simulations using reconfigurable circular patch antennas at the receiver of a MIMO system that employs minimum mean square error receivers for channel estimation. Channel capacity and Bit Error Rate (BER) results show the improvement offered relative to a conventional antenna selection technique for reconfigurable MIMO systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/146,332 filed Oct. 5, 2011, which is the National Stage ofInternational Application No. PCT/US2010/021917, filed Jan. 25, 2010,which claims the benefit of U.S. Provisional Patent Application Ser. No.61/147,365 filed Jan. 26, 2009, the disclosure of which is herebyincorporated by reference as if set forth in its entirety herein.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

Portions of the disclosure herein may have been supported in part by agrant from the National Science Foundation, Grant Nos. CNS-0322795,CNS-0322797 and ECS-0524200. The United States Government may havecertain rights in the invention.

TECHNICAL FIELD

The present invention relates generally to the field of multi-elementantenna systems. Specifically, the present invention relates to systemsand methods for efficiently using multi-element reconfigurable antennasin MIMO, SIMO and MISO systems.

BACKGROUND OF THE INVENTION

Recent studies have shown that employing reconfigurable antennas canimprove the gains offered by Multiple Input Multiple Output (MIMO),Single Input Multiple Output (SIMO) and Multiple Input Single Output(MISO) systems as explained in “Design and evaluation of areconfigurable antenna array for MIMO systems,” IEEE Transactions onAntennas and Propagation, vol. 56, no. 3, 2008, by D. Piazza, N. J.Kirsch, A. Forenza, R. W. Heath Jr., and K. R. Dandekar; “A MIMO systemwith multifunctional reconfigurable antennas,” IEEE Antennas andWireless Propagation Letters, vol. 5, no. 31, pp. 463-466, 2006, by B.A. Cetiner, E. Akay, E. Sengul, and E. Ayanoglu; “Multifunctionalreconfigurable MEMS integrated antennas for adaptive MIMO systems,” IEEECommunications Magazine, vol. 42, no. 12, pp. 62-70, 2004, by B. A.Cetiner, H. Jafarkhani, Jiang-Yuan Qian, Hui Jae Yoo, A. Grau, and F. DeFlaviis; “Maximizing MIMO capacity in sparse multipath withreconfigurable antenna arrays,” IEEE Journal of Selected Topics inSignal Processing, vol. 1, no. 1, pp. 156-166, 200, by A. M. Sayeed andV. Raghavan; and “Two port reconfigurable circular patch antenna forMIMO systems,” Proceedings of the European Conference on Antennas andPropagation, EUCAP, 2007 by D. Piazza, P. Mookiah, M. D'Amico, and K. R.Dandekar. These antennas adaptively change their electrical andradiation properties according to the propagation characteristics of thewireless channel in order to provide a strong channel between thetransmitting and receiving antennas in a given communication system.

To optimally use such reconfigurable antennas it is necessary to knowthe channel response between the transmitter and the receiver for eachantenna configuration as shown in “A reconfigurable multiple-inputmultiple-output communication system,” IEEE Transactions on WirelessCommunications, vol. 7, no. 5, 2008, by A. Grau, H. Jafarkhami, and F.De Flaviis. However, estimating the channel response for each antennaconfiguration at the transmitter and at the receiver as described in theabove-mentioned paper has been demonstrated to be power consuming and tohave a detrimental effect on the performance of the reconfigurable MIMO,MISO and SIMO systems. The negative effect of channel estimation on theperformance of the communication system increases proportionally withthe number of antenna configurations, reaching the point where thelosses, caused by imperfect channel estimation, may be higher than thecapacity gain offered by reconfigurable antennas.

In order to overcome this channel estimation problem, a method isproposed herein that allows both linear and non-linear multi-elementreconfigurable antennas to select the antenna configuration at thereceiver without any extra power consumption and modifications to thedata frame of conventional, non-reconfigurable MIMO, SIMO or MISOsystems. This configuration selection scheme does not aim to maximizethe throughput for each particular channel realization, but it selectsthe antenna configuration that, on average, increases the spectralefficiency of the communication link.

The adaptive algorithm presented below is shown to be effective forpattern reconfigurable antennas, though its use can also be extended toother classes of antennas. Pattern reconfigurable antennas are selectedbecause of their advantages in MIMO, SIMO or MISO communications withrespect to antennas that exploit space or polarization diversity.Pattern diversity antennas, similarly to polarization diversityantennas, allow system designers to reduce the antenna space occupied ona communication device, solving the size and cost constraints thatprevent the antennas from being placed far apart in conventional multielement antenna systems as taught in “Benefit of pattern diversity viatwo-element array of circular patch antennas in indoor clustered MIMOchannels,” IEEE Transactions on Communications, vol. 54, no. 5, pp.943-954, 2006, by A. Forenza and R. W. Heath Jr.

Also, unlike polarization reconfigurable antennas, patternreconfigurable antennas can be effectively used without the need forswitching antenna configuration at the transmitter and at the receiversimultaneously for polarization alignment. Moreover, patternreconfigurable antennas, unlike polarization reconfigurable antennas,allow for the generation of an ideal infinitive number of perfectlyuncorrelated patterns per antenna element in order to optimally tune thewireless channel for the highest spectral efficiency. A configurationselection scheme is proposed in accordance with the invention thatanalyzes the performance achievable with reconfigurable circular patchantennas as described in “Two port reconfigurable circular patch antennafor MIMO systems,” Proceedings of the European Conference on Antennasand Propagation, EUCAP, 2007, authored by D. Piazza, P. Mookiah, M.D'Amico, and K. R. Dandekar. As described in the D. Piazza paper, theseantennas are capable of dynamically changing their patterns by varyingthe radius of the circular patch. An analysis of the performance ofthese Reconfigurable Circular Patch Antennas (RCPAs), in terms ofergodic channel capacity and Bit Error Rate (BER), is conducted usingthe clustered channel model as taught in “TGn channel models,” IEEE802.11-03/940r4, 2004 by V. Erceg et al.

Through this approach the array configuration selection is directlylinked to i) the spatial characteristics of the wireless channel (anglespread of the power angular spectrum), ii) the levels of patterndiversity existing between the elements of the reconfigurable array,iii) the differences in radiation efficiency and input impedance betweenthe various antenna configurations, and iv) the average systemSignal-To-Noise-Ratio (SNR). An antenna selection scheme optimized formulti-element reconfigurable antennas is desirable and is describedherein.

SUMMARY

The proposed multi-element antenna selection scheme in accordance withthe invention selects the antenna array configuration for amulti-element reconfigurable transmitter and/or receiver antenna. Thesystem includes at least one of a transmitter antenna array and areceiver antenna array comprising multiple reconfigurable elements and aprocessor that implements software for the selection of an antenna arrayconfiguration and also builds a look-up table for at least one antennaarray configuration. Connective means such as PIN diodes, MEMS switches,FET transistors, variable inductors and/or variable capacitors areprovided that can be adjusted by the processor to reconfigure theantenna array configuration. The reconfigurable antenna arrayconfiguration can be, but is not limited to, a circular patch antennaarray. The receiver antenna array also can use linear or non linearreceivers to perform channel estimation. The transmit power can beequally distributed amongst array elements of the transmitter antennaarray or it can be adaptively distributed among the array elements ofthe transmitter antenna array.

The multiple reconfigurable elements can be used in various wirelesscommunication systems including, but not limited to: systems employingbeam forming, spatial multiplexing, space time diversity transmissionschemes, wireless local area networks, wireless personal area networks,wireless ad hoc networks, sensor networks, wireless body area networks,radar systems, satellite communications networks, 3G cellular networks,and/or 4G cellular networks.

The processor builds a look-up table of antenna array configurationswhich includes, but is not limited to, values representative of signalto noise ratio (SNR), angular spread (AS), reciprocal condition number(D_(σ)), reciprocal condition number of the transmit/receive correlationmatrices (D_(λ)), and delay spread (DS). Additional elements that can beused to build the look-up table include an electromagnetic clusteredchannel model, an electromagnetic ray tracing simulation, channelmeasurements or a system performance metric such as channel capacity,data transfer rate, bit error rate, packet error rate, or amount oftransmit power. The values can be estimated from information received indata packets of which part of each data packet is allocated for channelestimation.

The values for each array element are estimated for the various antennaarray configurations. The antenna array configuration is set for thetransmitter and/or the receiver antenna. The signal correlation forselected array configuration can also be determined and used todetermine the channel reciprocal condition number D_(λ). The channeltransfer matrix for the current array configuration can be determinedand used to determine the channel reciprocal condition number D_(σ).

The look-up table can be selected based on the array configuration usedto build the look-up table. The look-up table can also be selected basedon the direct measurement of SNR. The AS may be estimated using thereciprocal condition number information.

The array configuration preferably can be set to a reconfigurable arraywith total radiation pattern that guarantees quasi omni-directionalcoverage in a plane of an incoming signal. The selected antenna arrayconfiguration so selected can affect the shape of a radiation pattern,polarization of the radiation pattern, and/or separation between arrayelements of the antenna array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) illustrate schematics of the ReconfigurableCircular Patch Antenna (RCPA) with two (FIG. 1( a)) and three (FIG. 1(b)) antenna configurations, while FIGS. 1( c)-1(e) illustrate radiationpatterns excited in the azimuthal plane at the two ports of the RCPA fordifferent electromagnetic modes.

FIG. 2 illustrates RCPA radiation efficiency for different antennaconfigurations as a function of the substrate dielectric permittivity.

FIG. 3( a) illustrates channel capacity curves for three differentantenna configurations as a function of the angle spread (AS) for a 2×2MIMO system employing the RCPA-1 at the receiver, while FIG. 3( b)illustrates the percentage capacity improvement as a function of ASachievable when using the RCPA-1 in the same 2×2 MIMO system relative toa non-reconfigurable antenna system employing circular patch antennasoperating in modes TM₂₁, TM₃₁, and TM₄₁.

FIG. 4( a) illustrates channel capacity curves for three differentantenna configurations (TM₂₁, TM₃₁, TM₄₁) as a function of the anglespread for a 2×2 MIMO system employing the RCPA-2 at the receiver andFIG. 4( b) illustrates channel capacity curves for three differentantenna configurations (TM₂₁, TM₃₁, TM₄₁) as a function of the anglespread for a 2×2 MIMO system employing the RCPA-2 at the receiver for anideal RCPA with unitary radiation efficiency for all of the antennaconfigurations, where SNR=5 dB.

FIG. 5( a) illustrates channel capacity curves for three differentantenna configurations as a function of the angle spread for a 2×2 MIMOsystem employing the RCPA-1 at the receiver for SNR=0 dB and FIG. 5( b)illustrates channel capacity curves for three different antennaconfigurations as a function of the angle spread for a 2×2 MIMO systememploying the RCPA-1 at the receiver for SNR=20 dB.

FIG. 6 illustrates angle spread crossing points versus SNR forconfigurations TM₂₁×TM₃₁ and TM₃₁×TM₄₁.

FIG. 7 illustrates reciprocal condition number, D_(λ), as a function ofthe angle spread for the antenna configuration TM₂₁ used at the receiverin a 2×2 MIMO system.

FIG. 8( a) illustrates an achievable channel capacity as a function ofthe angle spread (AS) for a 2×2 MIMO system, and FIG. 8( b) illustratesthe percentage capacity improvement as a function of AS for the sameMIMO system that employs RCPA-1 at the receiver.

FIG. 9 illustrates achievable channel capacity as a function of theangle spread (AS) for a 2×6 MIMO with RCPAs (RCPA-1) at the receiver.

FIG. 10( a) illustrates BER versus SNR for a 2×2 MIMO system with RCPA-1at the receiver, and FIG. 10( b) illustrates the same BER curves as inFIG. 10( a) for an angle spread of 60°.

FIG. 11 illustrates a flow diagram that explains the selection of aproper group of look up tables based on the current array configuration.

FIG. 12 illustrates a sample look up table that can be used to selectthe antenna configuration knowing the SNR and D_(λ) for referenceconfiguration TM₂₁.

FIGS. 13( a)-13(c) illustrate a group of sample look up tables fordifferent reference antenna configurations.

FIG. 14( a) illustrates sample look up tables that can be used to selectthe antenna configuration knowing the SNR and D_(o) for referenceconfiguration TM₂₁ and FIG. 14( b) illustrates sample look up tablesthat can be used to select the antenna configuration knowing the SNR andDS for reference configuration TM₂₁.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

SIMO and MISO systems employ multi element antennas only at the receiverand at the transmitter respectively, while MIMO systems employ multielement antennas at both ends of the communication link. In thefollowing description, a MIMO communication system is considered as anexemplary embodiment, but those skilled in the art will also appreciatethat all the results apply also to SIMO and MISO systems.

Unlike conventional non-reconfigurable multi-element antenna systems, ina reconfigurable MIMO system, each antenna element of thetransmitting/receiving array is capable of changing its radiationpattern characteristics (i.e. pattern, polarization or both). Changingthe radiation properties of each antenna element has been shown to be aneffective technique to adapt to the changing conditions of the wirelesschannel in between the transmitter and the receiver. By properlyselecting the array configurations, it is possible to choose the channelscenario that allows for the highest throughput.

MIMO systems, employing reconfigurable arrays, are capable of Pdifferent configurations. Assuming a flat fading channel, the signalcollected at the receiver is related to the signal outgoing from thetransmitter through the relation:

y _(p,q) =H _(p,q) x _(p,q) +n _(p,q)

where y_(p,q)∈

^(N×1) is the signal vector at the receiver array, x_(p,q)∈

^(M×1) is the signal vector at the transmit antenna array, n_(p,q)∈

^(N×1) is the complex additive white Gaussian noise (AWGN) vector withvariance σ_(n) ² and H_(p,q)∈

^(N×M) is the channel transfer matrix. The subscripts p-th and q-threfer to the array configuration employed at the transmitter andreceiver multi element antenna respectively.

According to the Kronecker model, the transfer channel matrix H_(p,q),is defined as:

H_(p,q)=R_(RXq) ^(1/2)H_(w)R_(TXp) ^(1/2)

where RTXp and RRXq denote respectively the receive and transmit spatialcorrelation matrices for the p-th configuration of the receiving arrayand for the q-th configuration of the transmitting array, respectively.H_(w)∈

^(N×M) is a matrix of complex Gaussian fading coefficients. To performestimation of the channel response, H_(p,q), a pilot assisted estimationis considered that uses minimum mean square error (MMSE) receivers. Atraining sequence composed of L symbols is transmitted with a period ofK symbols, and used by the receiver to estimate the channel response. Itis common that the pilot signals assigned to the different transmitantennas are mutually orthogonal. This assumption implies that the totaltransmitted data per pilot sequence is equal to K−LM symbols.

The transmitted power is uniformly distributed across the M transmitantenna elements. The amplitude of the data symbol can then be expressedas:

$A = \sqrt{\frac{K}{\left( {\left( {K - {LM}} \right) + {\alpha^{2}L}} \right)M}\frac{P_{av}}{M}}$

where Pav is the average transmit power from all transmit antennas and αis a parameter that relates the amplitude of the data symbols to theamplitude of the training symbols A_(p), so that A_(p)=αA. Thepercentage of power allocated to the training symbols, α is then givenby:

$\mu = {\frac{L\; \alpha^{2}}{\left( {K - {LM}} \right) + {L\; \alpha^{2}}}{100\lbrack\%\rbrack}}$

For such communication systems, assuming perfect knowledge of thespatial correlation information at the transmitter and at the receiver,a lower bound of the achievable ergodic channel capacity may be definedas:

$C \geq {\frac{K - {LM}}{K}{E_{H_{p,q}}\left\lbrack {\log_{2}{\det \left( {I + {\frac{P_{av}}{M}{\hat{H}}_{p,q}{{\hat{H}}_{p,q}^{\dagger}\left( {\mathrm{\Upsilon} + {\sigma_{n}^{2}I}} \right)}^{- 1}}} \right)}} \right\rbrack}}$

where Ĥ_(p,q) is the estimated transfer channel matrix and fi is thecovariance matrix of the random vector H_(e)x_(p), with H_(e) being theMMSE estimation error on H_(w) (i.e. H_(e)=Ĥ_(w)−H₂); I is a N×Nidentity matrix and (†) denotes a complex conjugate transpose operation.Note that the term

$\frac{K - {LM}}{K}$

is introduced because L temporal signatures per each transmit antennaare allocated to the pilot. The covariance matrix, α, is defined as:

y=σ_(H) _(e) ²P_(αν)R_(RX) _(q)

where σ_(H) ² is the variance of the MMSE estimation error on H_(w). Forthis communication system σ_(H) _(e) ² is defined as:

$\sigma_{H_{e}}^{2}\left( \frac{1}{1 + \frac{\rho_{p}L_{p}}{M}} \right)$

where

$\rho_{p} = \frac{\alpha \; A}{\sigma_{n}^{2}}$

and L_(p) is the length of the sub-training sequence of L, allocated toestimate the channel transfer matrix for a particular antennaconfiguration at the transmitter and at the receiver L_(p)∈(0, L]). Notethat as a approaches 1 the ergodic channel capacity is that of a systemthat assumes perfect channel state information at the receiver (p-CSI).

One preferred embodiment of reconfigurable antennas includesreconfigurable circular patch antennas (RCPAs). The connective meansused to reconfigure the RCPA can include setting PIN diodes, MEMSswitches, FET transistors, variable inductors and/or variablecapacitors. RCPAs are antennas that can dynamically change the shape oftheir radiation patterns by varying the size of the circular patch. Eachantenna has two feed points and acts as a two element array. As depictedin FIG. 1, the two feed points on the antenna structure are separatedsuch that the radiation patterns excited at the two ports (port 1 andport 2) are orthogonal to each other. By simultaneously turning on andoff the switches located radially on the antenna, it is possible to varythe current distribution on the antenna structure and excite differentTM electromagnetic modes, each corresponding to a particular shape ofradiation pattern. The electric field components excited in the farfield by each port of the antenna for the n-th TM electromagnetic modeare defined as a function of the circular patch antenna radius, ρ, as:

${E_{\theta,1}^{(n)}\left( {\varphi,\theta} \right)} = {^{\frac{j\; n\; \pi}{2}}\frac{^{{- j}\; k_{o}d}}{d}\frac{V_{0}}{2}k_{0}{\rho \left\lbrack {{J_{n + 1}\left( {k_{0}{\rho sin}\; \theta} \right)} - {J_{n - 1}\left( {k_{0}{\rho sin\theta}} \right)}} \right\rbrack}{\cos \left\lbrack {n\left( {\varphi - \varphi_{0}} \right)} \right\rbrack}}$${E_{\varphi,1}^{(n)}\left( {\varphi,\theta} \right)} = {{- ^{\frac{j\; n\; \pi}{2}}}\frac{^{{- j}\; k_{o}d}}{d}\frac{V_{0}}{2}k_{0}{\rho \left\lbrack {{J_{n + 1}\left( {k_{0}{\rho sin}\; \theta} \right)} + {J_{n - 1}\left( {k_{0}{\rho sin\theta}} \right)}} \right\rbrack}\cos \; {{\theta sin}\left\lbrack {n\left( {\varphi - \varphi_{0}} \right)} \right\rbrack}}$${E_{\theta,2}^{(n)}\left( {\varphi,\theta} \right)} = {^{\frac{j\; n\; \pi}{2}}\frac{^{{- j}\; k_{o}d}}{d}\frac{V_{0}}{2}k_{0}{\rho \left\lbrack {{J_{n + 1}\left( {k_{0}{\rho sin}\; \theta} \right)} - {J_{n - 1}\left( {k_{0}{\rho sin\theta}} \right)}} \right\rbrack}{\sin \left\lbrack {n\left( {\varphi - \varphi_{0}} \right)} \right\rbrack}}$${E_{\varphi,2}^{(n)}\left( {\varphi,\theta} \right)} = {{- ^{\frac{j\; n\; \pi}{2}}}\frac{^{{- j}\; k_{o}d}}{d}\frac{V_{0}}{2}k_{0}{\rho \left\lbrack {{J_{n + 1}\left( {k_{0}{\rho sin}\; \theta} \right)} + {J_{n - 1}\left( {k_{0}{\rho sin\theta}} \right)}} \right\rbrack}\cos \; \theta \; {\cos \left\lbrack {n\left( {\varphi - \varphi_{0}} \right)} \right\rbrack}}$

where E_(θ,(1,2)) and E_(Φ,(1,2)) are the θ and Φ components of theelectric fields excited at port 1 and port 2 of the RCPA. J_(n) (k₀ρ sinθ) is the Bessel function of the first kind and order n; fi0 is thereference angle corresponding to the feed point location on the antenna;V₀ is the edge voltage at Φ=0; k₀ is the wave number; and d is thedistance from the antenna. Varying the radius of the antenna, differentelectromagnetic modes can be excited according to:

$\rho = \frac{X_{n}^{\prime}\lambda}{2\pi \sqrt{ɛ_{r}}}$

where ∈_(r) is the dielectric permittivity of the substrate, λ is thewavelength, and X_(n)′ is the first zero of the derivative of the Besselfunction Jn.

This embodiment includes RCPAs capable of exciting three differentelectromagnetic modes (i.e. configurations) at both ports: TM₂₁, TM₃₁and TM₄₁. The radiation patterns that are excited in the azimuthal planewith such RCPAs are shown in FIG. 1( c). The patterns excited at the twoports of the RCPA, for the same antenna configuration, are orthogonal toeach other and the variations between radiation patterns of differentRCPA modes occur in the number of lobes and in their beam width. Toquantify the level of diversity existing between radiation patternsexcited at the ports of the RCPA, one may use the spatial correlationcoefficient, {circumflex over (r)}_(j,k,l,m), defined as:

${\hat{r}}_{j,k,l,m} = \frac{\int_{4\pi}{{P(\Omega)}{{\underset{\_}{E}}_{j,k}(\Omega)}{{\underset{\_}{E}}_{l,m}(\Omega)}{\Omega}}}{\left\lbrack {{\int_{4\pi}{{P(\Omega)}{{\underset{\_}{E}}_{j,k}(\Omega)}}}^{2}{{{\Omega}{\int_{4\pi}{{\underset{\_}{E}}_{l,m}(\Omega)}}}^{2}{\Omega}}} \right\rbrack^{1/2}}$

where j and l define the array port and k and m the antennaconfiguration at the port j and l respectively. E _(j,k)(Ω) is theradiation pattern of the configuration k at port j over the solid angleΩ=(Φ,θ), P(Ω) is a probability density function that describes theincident multipath field distribution. For a rich scatteringenvironment, P(Ω) is uniformly distributed over

$\left( {{i.e.\mspace{14mu} {P(\Omega)}} = \frac{1}{4\pi}} \right).$

Table I shows the level of diversity existing between radiation patternsexcited at the two ports of the array for each antenna configuration({circumflex over (r)}_(j,k,l,m)), while Table II reports the level ofdiversity existing between the different antenna configurations({circumflex over (r)}_(j,k,l,m)).

TABLE I SPATIAL CORRELATION BETWEEN PATTERNS GENERATED AT TWO DIFFERENTPORTS OF THE RCPA FOR THE SAME CONFIGURATION-{circumflex over(r)}_(1,k,2,k) {circumflex over (r)}_(1,TM) ₂₁ _(,2,TM) ₂₁ {circumflexover (r)}_(1,TM) ₃₁ _(,2,TM) ₃₁ {circumflex over (r)}_(1,TM) ₄₁ _(,2,TM)₄₁ 0.63 0.63 0.63With this embodiment it is observed that the correlation values betweenradiation patterns excited at the two ports of the array are smallenough for all the configurations (≦0:7) to provide significantdiversity gain. Table II shows that the correlation between differentconfigurations is about 0:8 for all the states. Although this value islarge, the differences between the array configurations are high enoughto provide an improvement in terms of spectral efficiency and BER withrespect to non reconfigurable circular patch antennas.

TABLE II SPATIAL CORRELATION BETWEEN PATTERNS GENERATED AT THE SAME PORTOF THE RCPA-{circumflex over (r)}_(1,k,2,k) E_(1,TM) ₂₁ E_(1,TM) ₃₁E_(1,TM) ₄₁ E_(1,TM) ₂₁ 1 0.80 0.85 E_(1,TM) ₃₁ 0.80 1 0.81 E_(1,TM) ₄₁0.85 0.81 1

Differences between the various antenna configurations (andelectromagnetic modes) exists not only in the shape of the excitedradiation patterns, but also in the level of radiation efficiency, n,defined as:

$\eta = \frac{Q_{T}}{Q_{R}}$

where Q_(T) is the antenna total quality factor and Q_(R) is theradiation quality factor; Q_(T) takes into account dielectric,conduction and radiation losses while Q_(R) is a figure of merit foronly the radiation losses. They are defined, for a circular patchantenna, as:

$Q_{T} = \left( {\frac{1}{h\sqrt{\pi \; {uf}\; \sigma}} + {\tan \; \delta} + \frac{{h\; \mu \; {f\left( {k_{0}\rho} \right)}^{2}I_{1}}\;}{240\left\lbrack {x_{n}^{\prime 2} - n^{2}} \right\rbrack}} \right)^{- 1}$$Q_{R} = \left( \frac{240\left\lbrack {x_{n}^{\prime 2} - n^{2}} \right\rbrack}{h\; \mu \; {f\left( {k_{0}\rho} \right)}^{2}I_{1}} \right)$

where f is the frequency of operation, μ is the substrate dielectricpermeability, h is the substrate thickness, and σ is the conductivity ofthe material used to build the circular patch. tan δ is a figure ofmerit that takes into account the substrate losses and I₁ is defined as:

I ₁=∫₀ ^(π)[(J _(n+1)(k ₀ρ sin θ)−J _(n−1)(k ₀ρ sin θ))²++(cosθ)²(J_(n+1)(k ₀ρ sin θ)+J _(n−1)(k ₀ρ sin θ))²] sin θdθ

In FIG. 2 the radiation efficiency is reported as a function of thedielectric permittivity for different configurations of a RCPA matchedat 5:2 GHz and built on a substrate of thickness h=0:159 mm and tanδ=0:0009. It can be observed that the level of radiation efficiency isdifferent for each antenna configuration, as is true for most of theelectrically reconfigurable antennas proposed in the literature. For theRCPAs it is noted that the lower electromagnetic modes are moreefficient than the higher modes. Also, when the dielectric permittivityvalue increases, the radiation efficiency decreases. Two preferredembodiments of RCPAs for the selection algorithm differ in antennasubstrate and level of radiation efficiency. A summary of the maincharacteristics of these two antennas is reported in Table III below.

Each cluster is characterized by a mean angle of arrival (AOPA)Ω_(c),where Ω_(c)=(Φ_(c), θ_(c)) represents the solid angle consisting ofazimuth (Φ_(c)) and elevation (θ_(c)) components. Depending on thesystem bandwidth, the excess delay across different propagation pathsmay not be resolvable. In this case, multiple AOAs are defined with anoffset Φ with respect to the mean AOA of the cluster (Φ_(c)). This angleof arrival is generated according to a certain probability densityfunction (PDF) that models the power angular spectrum (PAS). Thevariance of the PAS is a measure of the angle spread (AS), σ_(Φ), of thecluster.

PAS is defined as P(Ω)=P_(Φ)(Ω)+P_(θ)(Ω), where P_(Φ) and P_(θ) are theangular power densities of the {circumflex over (Φ)} and {circumflexover (θ)} components of the incident field, respectively. It is alsoassumed that most of the scattered power propagates over the azimuthdirections. Therefore P(Ω)=Ω(Ω)*δ(Φ−Φ_(c))δ(θ−π/2), where * denotes theconvolution operator and Q(Ω) is generated according to the truncatedLaplacian distribution.

The spatial correlation between the k-th and m-th pattern configurationexcited at the j-th and l-th ports of multi element antennas, includingthe effect of the wireless channel, may be defined as:

$\begin{matrix}{r_{j,k,l,m} = {{\sqrt{\left( {1 - {S_{11j}}^{2}} \right){\eta_{j,k}\left( {1 - {S_{11}}^{2}} \right)}\eta_{l,m}} \times {\hat{r}}_{j,k,l,m}}=={\sqrt{\left( {1 - {S_{11_{j}}}^{2}} \right){\eta_{j,k}\left( {1 - {S_{11_{\iota}}}^{2}} \right)}\eta_{l,m}} \times \frac{\int_{4\pi}{{P(\Omega)}{{\underset{\_}{E}}_{j,k}(\Omega)}{{\underset{\_}{E}}_{l,m}^{\dagger}(\Omega)}{\Omega}}}{\int_{4\pi}{{P(\Omega)}{{{\underset{\_}{E}}_{ref}(\Omega)}}^{2}{\Omega}}}}}} & \; \\{\mspace{79mu} {where}} & \; \\{{\left\lbrack {\int_{4\pi}{{P(\Omega)}{{{\underset{\_}{E}}_{j,k}(\Omega)}}^{2}{\Omega}{\int_{4\pi}{{P(\Omega)}{{{\underset{\_}{E}}_{l,m}(\Omega)}}^{2}{\Omega}}}}} \right\rbrack^{1/2} = {\int_{4\pi}{{P(\Omega)}{{{\underset{\_}{E}}_{ref}(\Omega)}}^{2}{\Omega}}}},} & \;\end{matrix}$

is set with E _(ref)(Ω) being the electric field of a reference antennaconfiguration that is used as normalization factor for the spatialcorrelation coefficient. S₁₁ is the voltage reflection coefficients atthe antenna input ports and n is the antenna radiation efficiency.

The theoretical spatial correlation coefficients of a two port RCPA canbe expressed as:

${r_{l,m,l,m}\left( {\varphi_{c},\sigma_{\varphi}} \right)} = {\frac{\left( {1 - {S_{11_{l,m}}}} \right)\eta_{l,m}}{\left( {1 - ^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}}} \right)}\frac{\left( {n\; \sigma_{\varphi}} \right)^{2}}{1 + {2\left( {n\; \sigma_{\varphi}} \right)^{2}}} \times \times {\quad{{\left\lbrack {1 - ^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}} + {\frac{\sin^{2}\left( {n\; \varphi_{c}} \right)}{\left( {n\; \sigma_{\varphi}} \right)^{2}}\left( {1 - {^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}}{\cos \left( {n\; \pi} \right)}}} \right)}} \right\rbrack {r_{j,k,j,k}\left( {\varphi_{c},\sigma_{\varphi}} \right)}} = {\frac{\sqrt{\left( {1 - {S_{11_{l,m}}}} \right)\eta_{l,k}}}{\left( {1 - ^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}}} \right)}\frac{\left( {n\; \sigma_{\varphi}} \right)^{2}}{1 + {2\left( {n\; \sigma_{\varphi}} \right)^{2}}} \times \times {\quad{{\left\lbrack {1 - ^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}} + {\frac{\sin^{2}\left( {n\; \varphi_{c}} \right)}{\left( {n\; \sigma_{\varphi}} \right)^{2}}\left( {1 - {^{{- \sqrt{2}}{\pi/\sigma_{\varphi}}}{\cos \left( {n\; \pi} \right)}}} \right)}} \right\rbrack \mspace{79mu} {r_{j,k,l,m}\left( {\varphi_{c},\sigma_{\varphi}} \right)}} = {\frac{\sqrt{\left( {1 - {S_{11_{l,m}}}} \right){\eta_{l,m}\left( {1 - {S_{11_{j,k}}}} \right)}\eta_{j,k}}}{2}\frac{\left( {\sin \; n\; \varphi_{c}} \right)}{1 + {2\left( {n\; \sigma_{\varphi}} \right)^{2}}}}}}}}}}$

where it is assumed ∫_(4π)P(Ω)|E _(ref)(Ω)|²dΩ=1. The input impedance,efficiency and AS of the power angular spectrum describing the wirelesschannel act as scaling factors of the spatial correlation coefficient.Knowing the spatial correlation coefficient for each antennaconfiguration allows one to compute the transfer channel matrix H_(p,q).Only single sided correlated MIMO channels are considered herein. Inparticular, RCPAs are used only at the receiver while at the transmitterit is assumed RTX=I. This assumption is made because one may present anantenna configuration selection technique for the receiver,independently from the transmitter. The fact that RTX=I does not affectthe following analysis, which would not change for RTX 6=I.

The ergodic channel capacity achievable for each RCPA configuration canthen be calculated for the case of perfect channel state information atthe Receiver (α→∞), as discussed below. The following analysis isperformed in the single cluster channel model and the spatialcorrelation information is determined, using configuration TM₂₁ as areference antenna.

In another embodiment shown in FIG. 3( a) the average channel capacityachievable for some configurations of a reconfigurable circular patchantenna is reported as a function of the AS of the PAS, for a SNR=5 dB.The ergodic channel capacity values are averaged over all azimuthalangles (Φ_(c)∈[0.2π)) of the incoming PAS. Note that these results ofchannel capacity have been determined for a RCPA built on a RogersRT-duroid 5880 substrate and for a condition of perfect matching for allthe antenna configurations (RCPA-1). Table III provides a summary of theantenna related parameters. The achievable average channel capacityvaries as a function of the PAS angle spread. In particular, eachantenna configuration outperforms the others for a certain range ofangle spread. In FIG. 4 the same ergodic channel capacity of FIG. 3( a)is provided for a RCPA built on Rogers R03003 substrate (RCPA-2). Theparameters for RCPA-2 are also described in Table III. As shown in thistable, RCPA-2 is characterized by different values of radiationefficiency with respect to RCPA-1. A comparison of FIG. 3( a) with FIG.4 shows that the crossing points of the channel capacity traces vary asa function of the radiation efficiency of the configuration. This effectcan be better explained by looking at the average channel capacitycurves relative to an ideal RCPA having unit efficiency for all itsconfigurations (FIG. 4( b)). In this case, configuration TM₄₁outperforms the other configurations for low AS while for large AS allthe configurations perform the same. This happens because, at low AS,higher order modes are characterized by larger pattern diversity withrespect to lower modes, while at high AS, the level of pattern diversityis similar for all the antenna modes. On the other hand, the radiationefficiency, that is larger for lower order modes than for higher modes,determines configuration TM₂₁ to have the best performance at high AS,as depicted in FIG. 3( a) and FIG. 4( a). A similar conclusion could bedrawn if a variation in input impedance among the different antennaconfigurations.

TABLE III RCPAs CHARACTERISTICS RCPA-1 RCPA-2 substrate Rogers RT-duroid5580 Rogers R03003 ε_(r) 2.2 3 η_(TM) ₂₁ 0.94 0.88 η_(TM) ₃₁ 0.91 0.81η_(TM) ₄₁ 0.87 0.66 ρ_(TM) ₂₁ 0.33 λ 0.28 λ ρ_(TM) ₃₁ 0.45 λ 0.39 λρ_(TM) ₄₁ 0.57 λ 0.49 λ S₁₁ 0 0

These results demonstrate the possibility of selecting the antennaconfiguration at the receiver based on PAS angle spread knowledge, oncethe average system SNR is known. In FIG. 3( b) the percentage capacityimprovement achievable when using RCPAs relative to a non reconfigurableantenna system (i.e. fixed radius circular patch antennas operating inmode TM₂₁, TM₃₁ and TM₄₁). It can be noted that, for the system of FIG.3( b), using angle spread information to switch between configurationsleads to an average improvement of up to 5% with respect to a systemthat does not employ reconfigurable antennas.

The ergodic channel capacity of a MIMO system does not depend only onthe spatial correlation, but also on the system average SNR. In FIG. 5the average channel capacity achievable for the different configurationsof RCPA-1 is shown as a function of the AS of the PAS, for a SNR=20 dB(FIG. 5( a)) and SNR=0 dB. (FIG. 5( b)). The achievable channel capacityis, as expected, higher for the system with SNR=20 dB. than that of thesame system with average SNR=5 dB (FIG. 8( a)) and SNR=0 dB. The anglespread crossing points of the capacity curves, for different antennaconfigurations, shift with varying average system SNR. Using the samereconfigurable antenna, at SNR=20 dB, the AS crossing points values arehigher with respect to a system with SNR=0 dB.

In FIG. 6 the AS crossing points for configurations TM₄₁−TM₃₁ andTM₃₁−TM₂₁ are reported as a function of the system average SNR, for aMIMO system that employs RCPA-1 at the receiver. As the value of averagesystem SNR increases, the AS crossing point values increase as well.This effect can be explained because the channel capacity of MIMOsystems can be increased in two ways: i) increasing the system diversityand ii) increasing the amount of signal power received. The systemdiversity is reflected in the antenna correlation coefficient, while thesignal power received is influenced by the antenna efficiency and inputimpedance. Intuitively, at high SNR, since the received amount of powercan not be greatly modified by varying the antenna efficiency and inputimpedance, the level of antenna diversity is the dominant contributionto the achievable channel capacity. At low SNR, instead, small variationin antenna efficiency and input impedance can greatly affect the amountof received signal power, and therefore antenna efficiency and inputimpedance are dominant contributions on the channel capacity trend.

As shown in FIG. 5( a) at low SNR, the most efficient antenna (TM₂₁) haslarger advantage with respect to the other configurations. On the otherhand, at high SNR (FIG. 5( b)) the configuration with the lowest spatialcorrelation (TM₄₁) outperforms the others for more values of anglespread with respect to the same system at lower SNR.

Based on the above observations, it is possible to select the antennaconfiguration at the receiver based on knowledge of PAS angle spread andaverage system SNR.

For example, in this embodiment, a parameter of discrimination betweenthe different wireless channel scenarios is the reciprocal conditionnumber of the transmit/receive correlation matrices:

$D_{\lambda} = \frac{\lambda_{\max}}{\lambda_{\min}}$

where λ_(max) and λ_(min) are the maximum and minimum eigenvalues of thetransmit and receive correlation matrices.

In FIG. 7 a reciprocal condition number is plotted as a function of theangle spread of the PAS for a RCPA (RCPA-1) operating in mode TM₂₁. Foreach value of AS, there is a corresponding value of reciprocal conditionnumber, in particular for low values of AS the reciprocal conditionnumber is high and vice-versa. A map can be made given the averagesystem SNR and the values of AS that define a switching point betweentwo configurations (as shown in FIG. 3( a)) to a reciprocal conditionnumber. In Table IV, a mapping of reciprocal condition number tocorresponding values of AS regions is presented.

TABLE IV RELATIONSHIP OF ANGLE SPREAD TO RECIPROCAL CONDITION NUMBER FORSNR = 5 dB AS D_(λ) CONFIGURATION [0°, 13°) (11, ∞) TM₄₁ [13°, 23°) (5,11] TM₃₁ [23°, 360°) (0, 5] TM₂₁

Note that, given the results of FIG. 3( a), only three regions of D_(λ)need to be specified; each region corresponds to a particular antennaconfiguration at the receiver. This mapping procedure is necessary sincethe PAS angle spread is difficult to estimate, while thetransmit/receive spatial correlation matrices can be estimated usingstandard techniques. Note that the mapping procedure varies with theaverage system SNR, as explained below. Therefore an antenna table, likethe one of Table IV, need to be generated for each average SNR value.Alternatively, a two-entries table like the one of FIG. 12 can begenerated. According to this channel parameterization, it is thereforepossible to use second order wireless channel statistics together withthe average SNR, in order to determine receiver array configuration.Note that this approach allows the system to select the antennaconfiguration using the spatial correlation matrix of only one referenceantenna configuration without the need of estimating the channelresponse over each antenna configuration. This greatly simplifieschannel estimation in reconfigurable MIMO systems (discussed furtherbelow). In the example of Table IV and FIG. 12, the antennaconfiguration TM₂₁ has been selected as an arbitrary reference antenna.

Another parameter of discrimination between the different wirelesschannel scenarios is the reciprocal condition number of thetransmit/receive channel matrices, defined as:

$D_{\sigma} = \frac{\sigma_{\max}}{\sigma_{\min}}$

where σ_(max) and σ_(min) are the maximum and minimum eigenvalues of thetransmit and receive channel matrices. An example of look up table builtusing this parameter is shown in FIG. 14( a).

Another parameter of discrimination between the different wirelesschannel scenarios is the delay spread, DS. An example of look up tablebuilt using this parameter is shown in FIG. 14( b).

In accordance with the invention, a method is provided for selecting theantenna configuration, without estimating the channel transfer matrixfor each antenna configuration, but taking into account the effects ofboth directivity, radiation pattern shape and antenna gain. Such amethod 100 can be summarized with respect to FIG. 11 as follows:

Off Line Operations

1. Antenna look-up tables, like the one of FIG. 12, that maps theoptimal antenna configuration to the range of reciprocal numbers and/ordelay spread (DS) are built at step 102, one for each average SNR value,using, for example, the electromagnetic clustered channel model approachdescribed above. The information can be received in data packets whereeach data packet is allocated for channel estimation.

On Line Operations

2. The average system SNR is determined at step 104 and used to selectthe column of the antenna table generated for the current arrayconfigurations.

3. At least one of the spatial correlation matrix at the transmitter,Rt, and at the receiver, Rr, the transfer channel matrix H, and thedelay spread DS are determined for a reference antenna configuration.The spatial correlation matrix at the transmitter, Rt, and at thereceiver, Rr, are used to determine the channel reciprocal conditionnumber, D_(λ), and the transfer channel matrix, H, is used to determinethe reciprocal condition number, D_(σ), of the current arrayconfiguration at step 106.

4. The current array configuration determined at step 106 is then usedat step 108 to select the proper look up table (see, for example, FIGS.12-14).

5. At step 110, the proper column of the selected look up table isselected based on the measured SNR.

6. The information of the reciprocal condition number, D_(λ), D_(σ)and/or DS is used together with the antenna table at step 112 to selectthe antenna configuration at the receiver, which is set at step 114.

An example of look up table to be used with the method is shown in FIG.12 for reference configuration TM₂₁. It is also possible to define agroup of look up tables where each look up table corresponds to aparticular reference antenna configuration of the array as illustratedin FIGS. 13( a)-13(c).

The proposed selection algorithm requires the channel second orderstatistics to be constant over the configuration selection procedure.Estimation of the spatial correlation matrix could then be conductedusing standard techniques. Once the channel correlation is estimated andthe antenna configuration is selected, the L symbols of the pilotsequence can be used to estimate the channel for signal detection asdiscussed above.

Note also that variations of the method include the possibility ofselecting the antenna configuration using D_(σ) and DS information.Examples of look up tables built using D_(σ) or DS are shown in FIG. 14for the selection of the RCPA configurations of FIG. 1( b). Additionalvariations include use of electromagnetic ray tracing simulations todetermine values needed to populate the look-up table.

A method also may be provided for using sub-training sequences toestimate the transfer channel matrix for a particular antennaconfiguration. According to this approach, the achievable ergodiccapacity can then be computed. Note that contrary to this selectionapproach, the selection algorithm proposed herein always has Lp=Lindependently of the number of receiver antenna configurations. In thisway, a better estimation of the channel matrix can be obtained,resulting in better signal detection, and therefore, higher achievablechannel capacity and lower BER. The technique always selects the optimalantenna configuration based on the channel scenario that maximizes thereceive signal-to-noise ratio, while the proposed selection schemeselects the antenna configuration that on average increases the spectralefficiency of the communication link. The reconfigurable array has atotal radiation that guarantees quasi omni-directional coverage in aplane of an incoming signal.

The invention thus provides a method for estimating the channel only fora single antenna configuration rather than a selection scheme that needsto estimate the channel P times (one estimation per antennaconfiguration) for every training sequence.

In FIG. 8.(a) achievable channel capacity is shown as a function of theangle spread (AS) for a 2×2 MIMO system, with RCPA-1 at the receiver,that employs: (i) the proposed selection scheme including the effects ofimperfect channel estimation (proposed algorithm np-CSI), (ii) theproposed selection scheme assuming perfect channel estimation (proposedalgorithm p-CSI), (iii) an algorithm that selects the antennaconfiguration after estimating the channel for all possibleconfiguration including the effects of imperfect channel estimation(standard np-CSI); and (iv) a standard algorithm assuming perfectchannel estimation (standard p-CSI). The curves relative to the channelcapacity achievable with non reconfigurable circular patch antennasoperating in different modes assuming non-perfect channel estimation arealso reported. In FIG. 8( b) the percentage capacity improvement, as afunction of the angle spread (AS), is shown for the same 2×2 MIMO systemthat employs RCPA-1 at the receiver with the proposed selectionalgorithm relative to non reconfigurable antenna systems operating indifferent modes (proposed relative TM₂₁, TM₃₁ and TM₄₁) and RCPA systemthat selects the antenna configuration after exhaustively estimating thechannel for all possible configurations (proposed relative standard(np-CSI)), where SNR=5 dB.

In FIG. 9, the channel capacity achievable with the proposed selectionscheme is determined for a 2×6 MIMO system employing RCPAs only at thereceiver for a SNR=5 dB. The MIMO system configuration is shown in TableV. In this case, three RCPAs, built on Rogers RT-duroid 5880 substrate(RCPA-1), are used at the receiver with spatial separation of multiplewavelengths, such as to be uncorrelated one with the other. A total often possible array configurations can then be selected at the receiver(P=102), using a RCPA that is capable of switching between modes TM₂₁,TM₃₁ and TM₄₁. At the transmitter assume RTX=I. Since the number ofantenna configurations is higher than the 2×2 MIMO case (where P=3) thecapacity improvement achievable using the proposed selection scheme ishigher. The improvement is almost 20%. As explained above, the greaterthe number of array configurations, the worse the channel transfermatrix detection, and therefore the worse the channel capacity. Thisproblem is addressed by the proposed selection scheme that needs toestimate the channel for a single antenna configuration independently ofthe number of array configurations.

TABLE V MIMO SYSTEM CONFIGURATION 2 × 2 MIMO System 2 × 6 MIMO SystemRCPA type RCPA-1 RCPA-1 array configurations 3   10    states (P)μ_(proposed algorithm)  6% 11% α_(proposed algorithm) 0.44 0.58μ_(standard algorithm) 10% 20% α_(stantard algorithm) 0.61 0.88

An analysis of the proposed configuration selection algorithmperformance, in terms of BER, has been conducted for a 2×2 MIMO systememploying RCPA-1 antennas at the receiver. The modulation schemeconsidered is BPSK without any additional coding. BER values have beencalculated assuming perfect decoupling at the receiver of the two SingleInput Single Output (SISO) links comprising the 2×2 MIMO system.

The proposed algorithm achieves an appreciable gain with respect to astandard selection algorithm that selects the antenna configurationafter exhaustively estimating the channel for each configuration. Usingthe proposed algorithm the channel is better estimated than with thestandard algorithm. Specifically, in the proposed algorithm, thetraining sequence is entirely allocated to estimate the channel for asingle antenna configuration, instead of being allocated to estimate thechannel for all possible array configurations. This effect can be betterobserved by comparing the BER curve of a system with perfect channelestimation (standard algorithm p-CSI) with the BER curves of systemswith imperfect channel estimation (proposed algorithm np-CSI andstandard algorithm np-CSI).

Unlike a standard algorithm, the proposed configuration selection schemeestimates the channel for a single antenna configuration and thereforethe quality of channel estimation remains the same, independent of thenumber of array configurations. However, the diversity order of thesystem that uses the proposed algorithm is degraded with respect to asystem that uses the standard algorithm. This diversity orderdegradation is due to the fact that the proposed selection algorithmdoes not select the optimal antenna configuration for each particularchannel realization, but it selects the antenna configuration that, onaverage, increases the spectral efficiency of the communication link.

In FIG. 10( a), BER versus SNR is illustrated for a 2×2 MIMO system,with RCPA-1 at the receiver, that employs: (i) the proposed selectionscheme including the effects of imperfect channel estimation (proposedalgorithm np-CSI), (ii) an algorithm that selects the antennaconfiguration after estimating the channel for all possibleconfiguration including the effects of imperfect channel estimation(standard np-CSI), and (iii) a standard algorithm assuming perfectchannel estimation (standard p-CSI). The BER curves relative to nonreconfigurable circular patch antennas operating in different modesassuming non-perfect channel estimation are also reported at AS=10°.

In FIG. 10( b) the same BER curves are shown for an angle spread of 60°.A similar gain is achieved using the proposed algorithm with respect toa system that selects the antenna configuration after exhaustivelyestimating the channel for each configuration. However, it may beobserved that, different from the case of FIG. 10( a), at high AS theproposed algorithm always selects configuration TM₂₁. Recall that TM₂₁on average outperforms the other configurations, according to theresults above. Also, at high AS, the BER curve slope remains the samefor all the different configurations; therefore the level of diversityprovided by the different configurations is the same. The diversitylevel also affects the trend of the BER curves for the proposedalgorithm (proposed algorithm np-CSI) and the standard algorithm(standard algorithm np-CSI). Unlike the results from FIG. 10( a), athigh AS, the BER curve slope is the same for both systems; therefore, athigh AS, the two systems are characterized by the same diversity order.

The group of look up tables shown in FIGS. 13( a)-13(c) (one look uptable per array configuration) are pre-computed using the radiationpatterns of the reconfigurable antenna array at the receiver togetherwith a statistical model of the wireless channel (e.g. the clusteredchannel model). The RCPA radiation patterns and a clustered channelmodel are used to determine which array configuration achieves thehighest channel capacity for a particular range of SNR and angle spreadof the power angular spectrum (see FIG. 5 and FIG. 6). This informationis used to build the group of look up tables of FIG. 13( a)-13(c). SNRand D_(λ) are used as entries of the look up tables to select the arrayconfiguration at the receiver. Setting the antenna array configurationaffects the shape of a radiation pattern, polarization of the radiationpattern, and/or separation between array elements of the antenna array.The transmission power can be equally distributed amongst array elementsor adaptively distributed among the elements of the transmitter antennaarray.

The group of look up tables is stored on a processing unit. Assumingthat the array configuration at the receiver is set on configurationTM₂₁, the processing unit runs the following algorithm:

Scenario 1

-   -   1. The receiver waits to receive N data packets.    -   2. The last N data packets are used to measure the receive        spatial correlation matrix, R_(R), and SNR and to calculate        D_(λ). In the case of FIG. 13, SNR=6.5 dB and D_(λ)=27.    -   3. Since configuration TM₂₁ is in use, the processing unit        selects from the group of look up tables the look up table for        “reference configuration TM₂₁” (see FIG. 13).    -   4. The value of SNR is rounded to the closest integer shown in        the selected look up table. In the case of FIG. 13, SNR=5 dB.    -   5. SNR=5 dB and D_(λ)=27 are used as entries for the selected        look up table in order to define the array configuration to be        used at the receiver. In this case, configuration TM₃₁ is        selected.    -   6. Configuration TM₃₁ is set at the receiver of the        communication link and the algorithm starts again from 1.

In case the antenna configuration does not change after the processingunit completes all six steps, a variation from the example shown abovecould be the following:

Scenario 2

-   -   1. The receiver waits to receive N data packets.    -   2. The last N data packets are used to measure the receive        spatial correlation matrix, R_(R), and SNR and to calculate        D_(λ). In this case, SNR=6.5 dB and D_(λ)=5.    -   3. Since configuration TM₂₁ is in use, the processing unit        selects from the group of look up tables the look up table for        “reference configuration TM₂₁” (see FIG. 13).    -   4. The value of SNR is rounded to the closest integer shown in        the selected look up table. In this case, SNR=5 dB.    -   5. SNR=5 dB and D_(λ)=5 are used as entries for the selected        look up table in order to define the array configuration to be        used at the receiver. In this case, configuration TM₂₁ is        selected.    -   6. Configuration TM₂₁ is kept at the receiver of the        communication link.    -   7. The receiver waits to receive 1 data packet.    -   8. The last N data packets are used to measure R_(R), SNR and to        calculate D_(λ). In this case SNR=6.5 dB and D_(λ)=27.    -   9. Since configuration TM₂₁ is in use, the processing unit        selects from the group of look up tables the look up table for        “reference configuration TM₂₁” (see FIG. 13).    -   10. The value of SNR is rounded to the closest integer shown in        the selected look up table. In this case, SNR=5 dB.    -   11. SNR=5 dB and D_(λ)=27 are used as entries for the selected        look up table in order to define the array configuration to be        used at the receiver. In this case, configuration TM₃₁ is        selected.    -   12. Configuration TM₃₁ is set at the receiver of the        communication link and the algorithm starts again from step 1 of        SCENARIO 1.

In case the reconfigurable antenna system is employed at the transmitterand at the receiver, both the receiver and the transmitter will have aprocessing unit. Assuming that configuration TM₂₁ is initially in use atthe transmitter and at the receiver, SCENARIO 1 will change as follows:

Scenario 3.A (RX and TX)

-   -   1. The receiver waits to receive N data packets.    -   2. The last N data packets are used to measure R_(R), R_(T), and        the SNR at the receiver. D_(λ) at the receiver is calculated        from R_(R), while D_(λ) at the transmitter is calculated from        R_(T). In this case, SNR at the receiver=6.5 dB and D_(λ)=27 at        the receiver and D_(λ)=52 at the transmitter.    -   3. The receiver sends through a feedback channel the information        D_(λ)=52 to the transmitter processing unit.    -   4. The last transmitted N data packets are used to measure the        SNR at the transmitter. In this case, SNR=25 dB at the        transmitter.    -   5. Since configuration TM₂₁ is in use at the transmitter and at        the receiver, the transmitter and receiver processing units        select from the group of look up tables the look up table for        “reference configuration TM₂₁” (see FIG. 13).    -   6. The value of SNR is rounded to the closest integer shown in        the selected look up table. In this case, SNR=5 dB at the        receiver and SNR=25 dB at the transmitter.    -   7. SNR=5 dB and D_(λ)=27 are used as entries for the selected        look up table at the receiver in order to define the array        configuration to be used at the receiver. In this case        configuration TM₃₁ is selected.    -   8. SNR=25 dB and D_(λ)=52 are used as entries for the selected        look up table at the transmitter in order to define the array        configuration to be used at the transmitter. In this case        configuration TM₄₁ is selected.    -   9. Configuration TM₃₁ is set at the receiver of the        communication link, while configuration TM₄₁ is set at the        transmitter and the algorithm starts again from 1.

Depending on the user preference, it is possible to implement also thefollowing variation:

Scenario 3.B (RX and TX)

-   -   1. The receiver waits to receive N data packets.    -   2. The last N data packets are used to measure R_(R), SNR and to        calculate D_(λ). In this case, SNR=6.5 dB nd D_(λ)=27.    -   3. Since configuration TM₂₁ is in use, the processing unit        select from the group of look up tables the look up table for        “reference configuration TM₂₁” (see FIG. 13).    -   4. The value of SNR is rounded to the closest integer shown in        the selected look up table. In this case, SNR=5 dB.    -   5. SNR=5 dB and D_(λ)=27 are used as entries for the selected        look up table in order to define the array configuration to be        used at the receiver. In this case configuration TM₃₁ is        selected.    -   6. Configuration TM₃₁ is set at the receiver of the        communication link while the transmitter is still using        configuration TM₂₁.    -   7. The transmitter sends other M data packets.    -   8. The receiver waits to receive these M data packets.    -   9. The last M data packets are used to measure R_(T) and to        calculate D_(λ) at the transmitter. In this case, D_(λ)=52 at        the transmitter.    -   10. The receiver sends through a feedback channel the        information D_(λ)=52 to the transmitter processing unit.    -   11. The last transmitted M data packets are used to measure the        SNR at the transmitter. In this case, SNR=25 dB at the        transmitter.    -   12. Since configuration TM₂₁ is in use at the transmitter, the        transmitter processing units select from the group of look up        tables the look up table for “reference configuration TM₂₁” (see        FIG. 13).    -   13. The value of SNR is rounded to the closest integer shown in        the selected look up table. In this case, SNR=25 dB at the        transmitter.    -   14. SNR=25 dB and D_(λ)=52 are used as entries for the selected        look up table at the transmitter in order to define the array        configuration to be used at the transmitter. In this case,        configuration TM₄₁ is selected.    -   15. Configuration TM₄₁ is set at the transmitter and the        algorithm starts again from 1.

Note that the variation of SCENARIO 2 is applicable to SCENARIO 3.A andSCENARIO 3.B. In the above examples, look-up tables that use D_(λ) andSNR as entries were used to determine the antenna configuration to beused at the transmitter/receiver. As described above, other parameterslike D_(σ) and DS can be used as entries for these look up tables. Theabove examples can then be modified using tables similar to the onesshown in FIG. 14.

The diversity order of a system that adopts the proposed algorithm fallsin between the upper bound of a system that adopts a standardconfiguration selection algorithm and the lower bound of a system thatemploys non reconfigurable antennas. On the other hand, it is observedthat the proposed algorithm allows for better channel estimation (andthus, higher receiver SNR) than a standard configuration selectionscheme.

Note that the proposed method for selecting the antenna configurationcan be used with multi element reconfigurable antennas in MIMO, SIMO andMISO systems independently from the wireless communication system.Possible wireless communication systems that can take advantage of thisselection algorithm are wireless local area networks, wireless personalarea networks, wireless ad hoc networks, sensor networks, wireless bodyarea networks, radar systems, satellite communications, 3G and 4Gcellular networks, and wireless communication systems that employ beamforming, spatial multiplexing, or space time diversity transmissionschemes.

In one embodiment a method of using the channel model to build the lookup tables will need to be properly selected based on the particularapplication. Parts of such a system would include a transmitter antennaarray and a receiver antenna array (which can use both linear and nonlinear receivers) with multiple reconfigurable elements; a processor toimplement the selection software and create the look-up tables; PINdiodes, MEMS switches, FET transistors, variable inductors and/orvariable capacitors used to adjust the configuration based on theconfiguration data received from the processor through data packets. Thevalues reported in the look up tables of FIGS. 12, 13 and 14, forexample, have been determined using an electromagnetic cluster channelmodel defined for indoor MIMO wireless local area networks as stated inthe paper authored by V. Erceg et al, “TGn Channel models,” IEEE802.11.03/9490r4, 2004. The wireless channel model need to be selectedwith reference to the specific application in order to determine thevalues to be used in the look up tables.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

Therefore, it must be understood that the illustrated embodiment hasbeen set forth only for the purposes of example and that it should notbe taken as limiting the invention as defined by the following claims.For example, not withstanding the fact that the elements of a claim areset forth below in a certain combination, it must be expresslyunderstood that the invention includes other combinations of fewer,more, or different elements, which are disclosed above even when notinitially claimed in such combinations. A teaching that two elements arecombined in a claimed combination is further to be understood as alsoallowing for a claimed combination in which the two elements are notcombined with each other, but may be used alone or combined in othercombinations. The excision of any disclosed element of the invention isexplicitly contemplated as within the scope of the invention.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptually equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

What is claimed:
 1. A method of selecting an antenna array configurationfor a multi-element reconfigurable transmitter antenna and/or amulti-element reconfigurable receiver antenna in a transmission system,comprising the steps of: providing receiver look-up tables accessible bya receiver processor associated with said receiver antenna for at least1 of N antenna array configurations, said receiver look up tablesincluding values for antenna related parameters of discriminationbetween different wireless channel scenarios; said receiver processorestimating values for said antenna related parameters for at least onebut less than N antenna array configurations; and said receiverprocessor setting the antenna array configuration for the receiverantenna and/or the transmitter antenna based at least in part on saidestimated values and an array configuration retrieved from a receiverlook up table using the estimated values, said antenna arrayconfiguration for the receiver antenna and/or the transmitter antennabeing set independent of a transmission scheme or configuration adoptedby another antenna in said communication system.
 2. The method of claim1, wherein said antenna related parameters comprise at least one of asignal to noise ratio (SNR), an angular spread (AS), a reciprocalcondition number (D_(σ)), a reciprocal condition number of thetransmit/receive correlation matrices (D_(λ)) and a delay spread (DS).3. The method of claim 1, further comprising determining a signalcorrelation for a current array configuration and using the signalcorrelation to determine a channel reciprocal condition number D_(λ). 4.The method of claim 1, further comprising determining a channel transfermatrix for a current array configuration and using the channel transfermatrix to determine the channel reciprocal condition number D_(σ). 5.The method of claim 1, further comprising selecting the receiver look-uptable based on the array configuration that was used to build thereceiver look-up table.
 6. The method of claim 1, further comprisingselecting the receiver look-up table based on a direct measured signalto noise ratio.
 7. The method of claim 2, wherein the angular spread isestimated using D_(σ) and/or D_(λ).
 8. The method of claim 1, whereinbuilding the receiver look up tables comprises using an electromagneticclustered channel model.
 9. The method of claim 1, wherein building thereceiver look-up tables comprises using an electromagnetic ray tracingsimulation.
 10. The method of claim 1, wherein building the receiverlook-up tables comprises using channel measurements.
 11. The method ofclaim 1, wherein building the receiver look-up tables comprises using asystem performance metric.
 12. The method of claim 11, wherein theselected performance metric comprises channel capacity, data transferrate, bit error rate, packet error rate, or amount of transmit power.13. The method of claim 1, wherein the antenna array configuration setin the setting step is a reconfigurable array with total radiationpattern that guarantees quasi omni-directional coverage in a plane of anincoming signal.
 14. The method of claim 1, wherein setting of theantenna array configuration affects the shape of a radiation pattern,polarization of the radiation pattern, and/or separation between arrayelements of the antenna array.
 15. The method of claim 14, furthercomprising using connective means to reconfigure the antenna arrayconfiguration to the set antenna array configuration.
 16. The method ofclaim 15, wherein using connective means to reconfigure the antennaarray configuration comprises setting PIN diodes, MEMS switches, FETtransistors, variable inductors and/or variable capacitors.
 17. Themethod of claim 13, wherein the antenna array configuration is acircular patch antenna array.
 18. The method of claim 1, wherein saidestimated values are estimated from information received in data packetsof which part of each data packet is allocated for channel estimation.19. A transmission system for transmitting data from a first locationhaving a transmitter antenna and a transmitter processor to a secondlocation having a receiver antenna and a receiver processor, each ofsaid receiver antenna and said transmitter antenna comprising an arrayhaving multiple reconfigurable elements, wherein said receiver processorimplements receiver selection software for selecting an antenna arrayconfiguration for said receiver antenna array, said receiver selectionsoftware, when implemented, causing said receiver processor to buildreceiver look-up tables for at least 1 of N antenna arrayconfigurations, said receiver look up tables including antenna relatedparameters of discrimination between different wireless channelscenarios, to estimate values for said antenna related parameters for atleast one but less than N antenna array configurations, and to set theantenna array configuration for the receiver antenna and/or transmitterantenna based at least in part on such estimated values and an arrayconfiguration retrieved from a receiver look up table using theestimated values, said antenna array configuration for the receiverantenna and/or the transmitter antenna being set independent of atransmission scheme or configuration adopted by another antenna in saidtransmission system.
 20. The system of claim 19, wherein said antennarelated parameters comprise at least one of a signal to noise ratio(SNR), an angular spread (AS), a reciprocal condition number (D_(σ)), areciprocal condition number of the transmit/receive correlation matrices(D_(λ)) and a delay spread (DS).
 21. The system of claim 19, furthercomprising connective means that are adjusted by said processor toreconfigure the antenna array configuration to the set antenna arrayconfiguration.
 22. The system of claim 21 wherein said connective meanscomprises PIN diodes, MEMS switches, FET transistors, variable inductorsand/or variable capacitors.
 23. The system of claim 19 wherein thereconfigurable antenna array configuration comprises a circular patchantenna array.
 24. The system of claim 19, wherein said estimated valuesare estimated from information received in data packets of which part ofeach data packet is allocated for channel estimation.
 25. The system ofclaim 19, wherein the receiver antenna array uses linear or nonlinearreceivers to perform channel estimation.
 26. The system of claim 19,wherein transmit power is equally distributed amongst array elements ofsaid transmitter antenna array.
 27. The system of claim 19, whereintransmit power is adaptively distributed amongst array elements of saidtransmitter antenna array.
 28. The system of claim 19, wherein themultiple reconfigurable elements are used in a wireless communicationsystem.
 29. The system of claim 28, wherein the wireless communicationsystem employs beam forming, spatial multiplexing, or space timediversity transmission schemes.
 30. The system of claim 28, wherein thewireless communication system comprises a wireless local area network, awireless personal area network, a wireless ad hoc network, a sensornetwork, a wireless body area network, a radar system, a satellitecommunications network, a 3G cellular network, and/or a 4G cellularnetwork.